Slag wool, that is mineral wool produced from blasst-furnace slag, is normally made by blowing a blast of air or steam through a stream of molten blast-furnace slag. The result is a filamentary product which has good insulating properties and which furthermore is relatively fireproof. Normally this material is manufactured as a by-product of smelting. Hardened slag is typically heated to render it molten, and is thereafter blown or spun into filaments.
The fusing of blast-furnace slag in an arc furnace, normally with acidic or basic additives, is a relatively expensive process. Considerable energy must be used to fuse the slag, and to produce the steam normally needed to blow it into filaments.
Commonly owned U.S. Pat. No. 4,171,965 describes a method of and apparatus for producing so-called slag sand wherein a stream of freely flowing slag of relatively low viscosity is passed down through a trough and is mixed with a limited quantity of water so that its viscosity is only slightly increased, with the slag remaining plastic and below the threshold of pyroplasticity. The thus cooled slag is then mixed with additional water to stiffen it somewhat further, while still leaving it plastic, and is then poured onto a rapidly rotating vaned drum that reduces it to fine particles that are projected through the air to a collection location.
In accordance with this U.S. Pat. No. 4,171,965 the cooling of the slag stream, which effectively freezes it into a glassy condition, takes place in a trough such as described in commonly owned U.S. Pat. No. 4,115,089, whose entire disclosure is herewith incorporated by reference as is the entire disclosure of U.S. Pat. No. 4,171,965. The residence time of the slag in this trough, which is between 2 m and 6 m long, is between 5 sec and 8 sec, and can be controlled by an arrangement such as described in commonly owned U.S. Pat. No. 4,123,247, whose entire disclosure is also herewith incorporated by reference. In accordance with this U.S. Pat. No. 4,123,247 water is added only in a quantity of between 0.5 m.sup.3 and 0.7 m.sup.3 of water per ton of slag, and only in the upstream third of the trough.
Therefore in U.S. Pat. No. 4,123,247 the slag, which is still above the pyroplastic threshold, is mixed with water so that a portion of it is cooled still below that threshold. To this end the slag is poured between a pair of upright horizontally spaced and inclined slot-forming plates while being mixed with water that is fed in at a rate of between 0.3 m.sup.3 and 0.4 m.sup.3 of water per ton of slag. The water used is the process water used to cool the blast furnace and has a temperature well above 40.degree. C.
Thus in accordance with U.S. Pat. No. 4,171,965 it is possible to use an installation such as described in U.S. Pat. No. 4,115,089 but in an altogether different manner so as not to expand but merely to granulate the slag. The conversion of this system for granulation lies mainly in the manner in which the water is fed in, the amount of water fed in, and the speed at which the drum is rotating. Such conversion can be effected relatively easily in the known equipment.
The above-cited patent 4,171,965 describes an invention which, although parallel in certain manners to the other above-cited systems, can be used to produce slag granules which differ totally from the expanded slag produced by the other systems of U.S. Pat. Nos. 4,115,089 and 4,123,247. The progressive addition of small quantities of water does not produce expanded slag so that the resultant product has excellent hydraulic properties and does not have the crystalline structure that is so disadvantageous when using slag sand as, for example, in cement. This crystallization is normally caused by cooling the slag below a critical temperature with a large quantity of water relatively suddenly, that is by quenching. Such crystallization is a considerable problem in the production of slag sand. Indeed, the use of small quantities of water to produce slag sand with minimum crystallization as in accordance with the invention of U.S. Pat. No. 4,171,965 would normally seem to be impossible.
In the apparatus of that patent the drum is rotated at least 600 rpm and normally at approximately 100 rpm, so that if the drum is 50 cm in diameter it will have a peripheral speed of at least 94 meters/second. In the expanding operation described in the other U.S. Pat. Nos. 4,115,089 and 4,123,247 the drum normally rotates at a speed of between 250 rpm and 500 rpm, still having with the drum of 50 cm in diameter a peripheral speed of at least 40 meters/second. A water mist is sometimes generated between the drum and the collection location to increase the cooling.
In the normal slag granulation the slag sand usually is collected in a water-filled basin having at one end a drain. According to the invention of Patent 4,171,965 the slag sand is collected in foraminous substrate, on a conveyor belt formed as a screen such as described in Luxembourg patent applications 75,978 of Oct. 12, 1976 and 78,184 of Sept. 26, 1977, whose disclosures are also herewith incorporated by reference.